This invention relates to systems for the conveyance of storm water runoff or water of other origin which sometimes carries debris and sediment, and more particularly, to a facility for removing such debris and sediment from open channels in such a system.
Systems for conveying untreated water such as storm water runoff typically incorporate both open channels and enclosed conduits. The open channels are either waterways in essentially their natural condition, or waterways which have been improved by enlargement, straightening, paving, or other means to increase their hydraulic capacity and reduce erosion of their banks. The enclosed conduits include both short lengths of enclosed conduit (culverts) under embankments, and extended lengths of buried piping or other buried conduit (storm sewers, drains, and transmission mains).
Runoff from unpaved land that enters a water conveyance system typically carries amounts of debris (such as tree limbs and discarded manufactured objects) and sediment (soil particles and rocks). Quantities of debris and sediment transported by the water increase with the water flow rate, and high rates of flow can carry quantities of these materials that may substantially interfere with operation of the conveyance system. Debris tends to be trapped and accumulate wherever there is an obstruction in the flow path, or a reduction in the size of the flow path. Sediment tends to deposit wherever the velocity of flow is reduced, either by enlargement of the channel cross-section or by eddy effects. In either instance, the hydraulic capacity of the conveyance system is reduced where the debris or sediment accumulates, causing the water level upstream of that location to be higher at all rates of flow. This almost always leads to more frequent flood damage to property and will often cause increased hazard to human life.
Bridge crossings of stream channels, and culverts under embankments that cross stream channels, are hydraulic "choke points" that are especially susceptible to blockage by debris, and present an especially high risk to people's lives and property. Obstruction of a bridge opening or culvert by debris or sediment reduces its hydraulic capacity. During periods of high flow, this causes upstream water levels to rise to the level of the bridge or the top of an embankment, and the water to flow over the roadway. The force of the current can wash vehicles off of the roadway into the stream, and can wash out the bridge or embankment.
The debris and sediment that accumulates in storm sewers also reduces their hydraulic capacity. This causes low-lying areas in a watershed to flood during more frequent, less intense rainstorms. Removing debris and sediment from an enclosed conduit is generally much more difficult and costly than removing these materials from an open channel, because of the difficulty of access to an enclosed conduit.